1. Field of the Invention
The present invention generally relates to a library apparatus employing a magnetic tape cartridge or an optical disk or the like as a storage medium, and more particularly to a library apparatus in which a plurality of cell drums are placed along running rails and further the medium is conveyed between the cell drum and a deck unit by two accessors.
2. Description of the Related Art
In recent years, with increase in the number of media used in an external storage such as a magnetic tape unit (MTU), an optical disk or the like, the enhancement of the efficiency in operation, the maintainability and the reliability of a computer system have been required. With this end in view, there has been provided a library apparatus for keeping media and for performing an automatic operation of conveying a medium to an external storage.
FIG. 1 is a schematic perspective view of a conventional library apparatus. This library apparatus is partitioned into a plurality of frames. Conversely, the library can be constituted by combining these frames with one another. Further, a cell block 1000 having a large number of cells for keeping media is placed in the library apparatus. For example, a large number of medium cartridges, each of which contains magnetic tape, are accommodated therein. The positions of the cells of the cell block are defined as cell addresses. Moreover, in each of the frames, rails 1014 are laid on the bottom surface thereof from the left side to the right side thereof. Furthermore, accessors 1012-1 and 1012-2, which are robots, move on the rails 1014. Further, deck units 1016-1 and 1016-2 and control units 1018-1 and 1018-2 are placed across the rails 1014 from the cell block 1000. Moreover, a plurality of recording/reproducing units are provided in each of the deck units 1016-1 and 1016-2. The recording/reproducing units are loaded with the cartridges conveyed by the accessors 1012-1 and 1012-2 from the cell block 1000 and are operative to read data therefrom and write data thereto. Additionally, a medium inlet/outlet port 1015, from which the cartridges are taken in and out, is provided in a front portion of the frame of the library apparatus.
However, in the case of such a conventional library apparatus, the medium cartridges are enclosed in each of the cell blocks. Thus, to increase the number of the cartridges accommodated in the apparatus, the cell blocks are aligned along the rails. However, there is a limit to a space in which the cell blocks are placed. Consequently, there has been a problem in that the conventional library apparatus can not fully cope with the increase in number of the cell blocks to be accommodated therein. Moreover, in the case where the number of accommodated cartridges is increased by aligning the cell blocks, the running distance of the accessor be comes long and it takes time to convey the cartridge. This results in causing a problem in that the speed-up of the conveying of cartridges is hindered. Furthermore, in the case where the medium is conveyed by using the two accessors in the library apparatus, the medium conveying operation of one of the accessors should be controlled by retreating and making the other of the accessors wait if the moving ranges of the accessors overlap with each other. Thus, it takes time more for that to perform the processing. Regarding this problem, in the case where a cell drum is placed instead of the cell block, the accessor is sometimes made to wait while the cell drum is rotated until an indicated cell face a position from which the accessor can take the cartridge. Further, the time, during the accessor waits the rotation of the cell drum, should be minimized.
Additionally in the cases of the library apparatuses developed in recent years, the density of cells is increased with increase in amount of contained media. As a result, the accuracy of positioning at the time of taking a medium in and out of a cell by means of the robot hand of the accessor should be improved. For this purpose, it is desirable to improve the detection precision of a sensor for detecting the position of a cell, which is mounted on the robot hand. The position of a cell is detected by irradiating the cell with light issued from a light emitting portion and using a photodetector for detecting light reflected by the cell. The photodetector, however, is operative to detect feeble scattering light. Therefore, the sensitivity of the sensor should be heightened considerably, with the result that the signal-to-noise ratio (S/N) thereof is degraded. Accordingly, there has been a problem in that the sufficient detection precision of the sensor can not be obtained. Further, the position of a cell is sensed by detecting a cell edge by use of an optical sensor while moving the accessor in a direction. Thus, there has been another problem in that a steady-state error occurs. Moreover, the sensitivity of the sensor mounted on the robot hand should be regulated in a state in which the accessor has been moved to the front of a cell. It is, therefore, very troublesome to regulate the sensitivity of the sensor. Furthermore, in the case where the cell drum is used instead of the cell block, the drum is rotated only by an angle corresponding to a place, on which a cell is provided, in the cylindrical surface of the drum. However, there is a difference between a designed angle of rotation and the angle of rotation, at which the cell is actually placed, in front of the accessor. Thus, there has been the fear that the medium cartridge may not be well taken in and out of the cell by use of the robot hand.
The controller of the accessor has a table for translating a cell address, which is indicated by a moving command issued by a host computer, into X- and Y-coordinates of the accessor. Values contained in this translation table are actually measured in a measuring operation of the accessor and are then stored in a read-only memory (ROM). Simultaneously, such values are stored in a floppy disk or the like as backup data. Further, when performing a power-on starting, the value serving as reference data, which is stored in the floppy disk, is compared with that stored in the ROM. If not matched, the re-acquisition of the value of the cell address should be performed by conducting a measurement thereof by use of the accessor as an operation to be performed when data exception occurs. Thus, there has been a problem in that it takes time to start up the apparatus.
In the case of the accessor used in the conventional library apparatus, only one running motor is mounted in a rail running portion thereof. Thus the accessor has reached the limits to the speed-up of the speed at which the medium is conveyed. Moreover, when a failure occurs in the running motor, the accessor does not work at all. Therefore, there has been a problem in that in such a case, the library apparatus falls into an unoperatable condition.
Meanwhile, the accessor of the conventional library apparatus for containing magnetic tape or the like horizontally moves along horizontal guide rails. Further, the accessor is provided with a robot hand mechanism which can move vertically along guide rails formed in a vertical column. Moreover, the robot hand mechanism has a hand unit provided with upper and lower hands for holding a cartridge. The hand unit requires performing operations of opening and closing (namely, joining and unjoining) the upper and lower hands and an operation of sliding back and forth. The lower hand is movably attached to the upper hand. Further, the recording medium cartridge is held by the upper and lower hands. Moreover, the lower hand is pushed by a plurality of springs in a direction in which the lower hand comes closer to the upper hand, namely, in a direction in which the upper and lower hands are closed or joined. A conventional hand opening/closing mechanism comprises a reversible motor, a first gear fixed to the output shaft or axis of the motor, a second gear which meshes with the first gear, and a roller rotatably mounted on a shaft displaced from the axis of rotation of the second gear. The roller is brought into abutting engagement with a lower-hand pushing-down face. When the motor is turned in a direction, the roller moves eccentrically to the first gear to thereby push down the lower hand. Thus the hands are opened or unjoined. When the motor is reversed, the lower hand is made to come close to the upper hand by the force of the springs. As a result, the hands are closed or joined. Namely, in the case of the conventional hand opening/closing mechanism opens and closes (that is, joins and unjoins) the hands by rotating the motor in normal and reverse directions, respectively.
The conventional hand opening/closing mechanism, however, requires controlling the motor in such a manner as to rotate the second gear about 80 degrees and then hold the second gear at such a position, thereby pushing down the lower hand to a predetermined position. Thus, the positioning accuracy should be sufficiently high to such an extent that the roller can be turned to the predetermined position and then be held thereat. Consequently, there have been problems in that it is difficult to control the motor in such a way as to prevent the roller from being pushed up by the springs and that the conventional apparatus is susceptible to variations in quality or accuracy of components.
If running the library apparatus for a long time period, the head of the drive unit becomes contaminated. Accordingly, the head needs to be cleaned. The cleaning of the head is performed by causing the accessor to insert a cleaning cartridge into the drive unit. In the case of the conventional library apparatus, the cleaning of the head is performed as follows. Namely, the host computer connected thereto first identifies a cell which contains a cleaning cartridge. Then, in response to an instruction issued from the host computer, the accessor conveys the cartridge to the drive unit. Subsequently, an operation of cleaning the head is carried out. The accessor, however, is not provided with a mechanism for discriminating between an ordinary cartridge and a cleaning cartridge. Thus, the accessor inserts a cartridge into the drive unit whereupon the kind of cartridge is identified. Alternatively, a cartridge is conveyed to a separately provided cartridge discriminating mechanism which identifies the kind of cartridge. Accordingly, there has been a problem in that it takes time to discriminate a cleaning cartridge from an ordinary cartridge. Moreover, in the case of the conventional apparatus, the host computer should preliminarily identify a cell which contains a cleaning cartridge. This has caused an increase in load on the host computer.
Furthermore, in the case of the conventional library apparatus, the bottom plate of each cell is upwardly inclined only by a predetermined angle in order to prevent a cartridge, which is contained in each cell, from popping out therefrom. In the case of such a library apparatus, the base and the hand unit of the accessor are also tilted according to the angle of inclination of the cell. Thus, there has been a fear that when a power supply for supplying electric power to a motor for moving the hand unit back and forth is shut down by some cause while the accessor moves, the hand unit might pop out to the front of the accessor owing to its own weight and interfere with peripheral equipment or the like and consequently, the hand unit and/or the peripheral equipment might be damaged.